1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for selectively sealing the wellbore. More particularly, the apparatus relates to a seal that is convertible to a flow control seal. More particularly still, the apparatus relates to a seal having a plug and a valve, the valve being held in an open position upon run in and setting of the seal. More particularly still, the apparatus relates to a seal having a plug and a valve, the plug is removed when desired to allow the valve to control flow through the seal.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the wellbore. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
There are various downhole operations in which it may become necessary to isolate particular zones within the well. This is typically accomplished by temporarily plugging off the well casing at a given point or points with a bridge plug. Bridge plugs are particularly useful in accomplishing operations such as isolating perforations in one portion of a well from perforations in another portion or for isolating the bottom of a well from a wellhead. The purpose of the plug is simply to isolate some portion of the well from another portion of the well. Bridge plugs do not allow flow past the plug in either direction. In order to reestablish flow past a bridge plug an operator must remove and/or destroy the bridge plug by milling, drilling, or dissolving the bridge plug.
During a fracturing or stimulation operation of a production zone, it is often necessary to seal the production zone from wellbore fluids while allowing production fluids to travel up the wellbore and past the seal. Frac plugs are designed to act as a seal and to provide a fluid path therethrough. Frac plugs typically have a one way valve which prevents fluids from flowing downhole while allowing fluids to flow uphole. In operation, a frac plug is installed above the zone that has been fractured (frac'd) or treated. This seals the treated zone from the uphole wellbore fluids while allowing any production fluids to flow through the frac plug. After the frac plug is set, an operator may treat an uphole zone without interfering with the previously treated downhole zone. Once the uphole zone is treated, a second frac plug may be set above it. This process may be repeated until all, or a select number, of the production zones in the wellbore have been treated.
In some instances, it may be desirable to seal a treated lower zone from flow in both directions while treating an upper zone. In particular, it is often desirable to reduce the wellbore pressure above the pressure-charged treated lower zone by setting a pressure isolation device and then bleeding off wellbore pressure at the surface. This is desirable for safety reasons as well as providing a negative pressure test on the plug, which is set above the treated zone. This is not possible using a frac plug. Instead, this requires setting a bridge plug above the treated zone. The pressure above the bridge plug is then bled off. The upper zone may then be treated while flow to the lower zone is prevented. After the upper zone has been treated, the bridge plug is removed and a frac plug is set in its place. The removal of the bridge plug and setting of the frac plug generally requires separate trips downhole. Each trip adds to the expense of the operation. Further, the time required to set the frac plug after the bridge plug is removed may cause damage to the lower zone due to wellbore pressure entering the treated zone.
There is a need, therefore, for a bridge plug which can be converted to a frac plug. There is a further need for the bridge plug to have a valve which is mechanically held in the open position until the bridge plug is converted to a frac plug.